Fiber cement is commonly used as siding in the housing industry due to the many advantages it provides. For instance fiber cement does not rot or deteriorate and therefore, provides long-term durability. Further, fiber cement is non-combustible.
The production of fiber cement begins with a mixture of a special type of cement, silica, and fibers. Although asbestos fibers were once used in the production of fiber cement, now plastic fibers, carbon fibers, and wood fibers are more commonly found in fiber cement. The mixture is approximately 80% cement and silica and 20% fiber. The fibers act in a manner similar to reinforcing rods used in concrete.
Water is added to the cement, silica, and fiber mixture to form a slurry with a consistency ranging from 4% to 12% depending on the rate of production and type of product produced. The slurry has an unusually high alkalinity ranging from a pH of 9.0 to 13.0. The slurry is added to an individual cylinder or a cascading arrangement depending on the type of machine. In the cascading arrangement, the slurry is fed to the first cylinder and overflows from the first cylinder to the next cylinder. FIG. 1 depicts an example of a cylinder.
After which, the slurry is formed into a sheet on a wire-covered cylinder mold by a head-differential between the inside and outside of the cylinder mold. The sheet is couched to a felt that contacts the top of the cylinder mold and carries the wet fibrous mat to the next cylinder, if necessary, where it picks up another ply or layer.
Due to the tenderness of the sheet formed in the early stages of formation, a great deal of vacuum equipment is used in a vacuum zone to remove water from the formed sheet rather than applying pressure. Once the sheet is sufficiently set, a press is used to remove free water.
FIG. 1b is a plan diagram of an illustrative two-cylinder fiber cement production machine. The machine includes a forming section 70 for generating fiber cement in a layered form, a mandrel 80 for accumulating the generated layered material, and a conveyor section 90 for transferring the accumulated material from the mandrel to other locations.
As can be seen from FIG. 1b, the forming section includes a felt belt 72, which moves in a generally counterclockwise direction within the figure, and two cylinders 74, each of which may be the same or similar to the cylinder shown in FIG. 1. As the felt passes the first cylinder a layer of material is deposited on the felt's underside. As the felt passes the second cylinder a second layer is added. Movement of the felt is controlled through a system of rollers that include: a number of rollers 76 for guiding the felt along its path, a main drive roller 76′ for driving the belt along its direction of motion, and a float roll 76″ for maintaining desired belt tension. The felt is rinsed at various points along its path by several rinsing heads 77. Drying of the felt at desired points is achieved through the use of vacuum boxes 78.
The multi-layered material created on the felt of the forming section is accumulated on mandrel 80 and transferred from mandrel 80 by conveyor section 90.
The conveyor section includes a conveyor belt 92, a series of rollers 94 for controlling movement of the conveyor belt, a float roll 94′ for controlling the conveyor belt tension.
Having discussed the production of fiber cement in general, the production of paper will now be discussed in general.
The production of paper begins with the processing of wood. Wood is chiefly composed of two major substances; both are organic, that is, their molecules are built around chains and rings of carbon atoms. Cellulose, which occurs in the walls of the plant cells, is the fibrous material that is used to make paper. Lignin is a large, complex molecule; it acts as a kind of glue that holds the cellulose fibers together and stiffens the cell walls, giving wood its mechanical strength. In order to convert wood into pulp suitable for making paper, the cellulose fibers must be freed from the lignin. FIG. 2 depicts an example of the pulping process. In mechanical pulping this is done by tearing the wood fibers apart physically to create groundwood pulp, leaving most of the lignin intact in the pulp. The high lignin content of groundwood pulp leaves the paper products weak and prone to degradation (e.g. yellowing) over time. Mechanical pulp is used principally to manufacture newsprint and some magazines.
In most pulp production lignin is separated from the fibers chemically. For example, in the kraft process, wood chips are heated (“cooked”) in a solution of sodium hydroxide and sodium sulfide. The lignin is broken down into smaller segments and dissolves into the solution. In the next step, “brownstock washing,” the breakdown products and chemicals are washed out of the pulp and sent to the recovery boiler. Kraft unbleached pulp has a distinctive dark brown color, due to darkened residual lignin, but is nevertheless exceptionally strong and suitable for packaging, tissue and toweling.
For brighter and more durable products the pulp must be bleached. In the bleaching process, the color in the residual lignin is either neutralized (by destroying the chromophoric groups) or removed with the lignin. This process traditionally has been accomplished for kraft pulp by chlorine bleaching, usually followed by washing and extraction of the chemicals and breakdown products. This process is not much different than washing clothes, the stains imbedded in cloth fibers are either neutralized by bleach, or broken down and washed out.
In current pulp production processes, the lignin solution typically undergoes two or more separate washing operations. For example, the groundwood or wood chips are first processed with chemicals under pressure and temperature, either usually by the kraft process or by the sulfite acid process. In either process, digestion dissolves the lignins thereby freeing the fibers and placing the lignin components into solution. In both processes the resulting liquid is dark in color, and the residual liquid which does not drain from the pulp and the remaining contaminants must be washed from the pulp. Further, it is desirable to recover spent liquid at as high a concentration as practical to minimize the cost of the subsequent recovery of chemicals.
Brown pulp which has been so washed retains a definite brown color and the pulp which remains is usually too highly colored for making white paper. Also, if any lignin is present, paper made from such pulp may not have a high degree of permanence and will yellow in time. Therefore, it is common and conventional to apply a bleaching process to the pulp, not only to improve whiteness, but to improve permanence of the whiteness.
The bleaching commonly is performed in a chlorination stage by applying water in which chlorine gas has been dissolved. Other bleaching processes may be used, such as a sodium hydrosulphite process, as is well known in the art. Three chemicals that are commonly used in current bleaching operations are sodium hydroxide (NaOH), chlorine dioxide (ClO2) and hydrogen peroxide (H2O2). Bleaching may not be accomplished in a single stage and may be performed in two or more stages, each followed by washing. After bleach treatments, the pulp is subjected to a washing action to remove the water that contains the spent bleaching agents and dissolved lignin.
For example, one method of removing the water containing the spent bleaching agents and dissolved lignin involves the use of a belt-type pulp washing machine that includes a dewatering stage (or “formation zone”) and multiple counter-current washing stages (or collectively “displacement zone”). The formation zone of the machine employs an endless moving foraminous belt which extends about a breast roll defining an on-running end and a couch roll defining an off-running end, with a generally horizontal upper run of the belt extending between the rolls. A series of suction boxes located underneath the belt provide for initial dewatering of the pulp in the formation zone, and combine with a series of showers to provide washing and dewatering in the displacement zone.
As shown in FIG. 3, downstream from the belt washer a series of washing zones or stages to which a washing liquid is applied from above for drainage through the pulp mat. As can be seen in FIG. 4, the freshest or cleanest washing liquid is applied to the zone nearest the last working stage and the liquid drained through the mat at that zone is collected and delivered to the immediately preceding washing zone. This is repeated from zone to zone, so that the cleanest pulp is treated with the cleanest water, and the dirtiest pulp is treated with the dirtiest water.
Both fiber cement production and paper pulp production, utilize a drum covered by a two-layer drum cover constructed of two independently woven metal wires. The first wire, which is more commonly referred to as a “backing” wire, is attached to the drum first and is typically constructed of a high-grade bronze warp and a lower grade bronze shute. The second wire, which is more commonly referred to as a “face” wire, is the main support of the two-layer drum system and consists of metal alloys having a higher grade than the metal alloys used for the backing wire.